Breaking five weeks of silence, Catherine T. Geoghan, the sister of defrocked priest John J. Geoghan, today will express concerns about what she sees as bias in the official investigation into her brother's death, according to a person who has read a two-page statement Geoghan plans to release.

''She wants the truth to come out about how her brother was killed, and she's worried it won't,'' said the person, who has seen the statement and asked not to be identified. ''The process is not going smoothly and she is just not happy.''

In the statement, Geoghan, 69, cites as an example statements she says have been made by some prisoners who were present when her brother was killed in prison. Those inmates say they have been told by state Department of Correction officials to keep quiet about possible lapses by prison officials, according to the person who has read the statement.

John Geoghan, 68, was beaten and strangled Aug. 23 inside his cell in the protective custody unit of the maximum-security Souza-Baranowski Correctional Center in Shirley.

Catherine Geoghan's statement is to be released today by Massachusetts Correctional Legal Services, an agency that provides lawyers for prisoners.

How Geoghan, whose prison record was devoid of violent incidents, wound up in one of the state's highest-security facilities is one of the questions being investigated by a three-member panel appointed to examine the death of the former priest whose actions helped trigger the clergy sexual abuse scandal.

Lawyers who represented Geoghan say guards at the medium-security Concord state prison, where he was jailed before being transferred to Souza-Baranowski, constantly harassed and abused Geoghan and filed bogus disciplinary reports in an effort to get him transferred to the more dangerous maximum-security facility.

At Souza-Baranowski, authorities said, Geoghan was killed by Joseph L. Druce, 38, who was serving a life sentence for beating and strangling a Gloucester man. During his murder trial, Druce claimed that victim had made a sexual advance.

The three-member panel investigating the Geoghan case is chaired by Mark Delaney, a State Police major, and includes Mark Reilly, the Department of Correction's chief investigator, and George Camp, a prison consultant and executive director of an organization that represents the interests of the nation's top prison officials. The panel was established two days after Geoghan's death at the urging of Governor Mitt Romney, who called the killing ''a failure of government.''

The panel is to report back in November to Edward A. Flynn, the state secretary of public safety who oversees the State Police and the Department of Correction.

Since Geoghan's death, his sister has closely followed developments in the case, according to the person who has read her statement. A group of state legislators, prisoners' rights advocates, and civil rights lawyers publicly called on Flynn to add independent members of the panel.

Meanwhile, the cochairmen of the Legislature's Joint Committee on Public Safety has scheduled a public hearing on the Geoghan case for Oct. 28.

A spokeswoman for Flynn did not return telephone calls yesterday. Justin Latini, a spokesman for the Department of Correction, declined to comment on any aspect of the investigation.

John Geoghan was serving a nine- to 10-year prison sentence for molesting a 10-year-old boy, but allegations that Geoghan sexually assaulted nearly 150 children, mostly boys, helped spark the scandal that rocked the Catholic Church. Catherine Geoghan was her brother's closest confidante and most steadfast supporter, and in 2000 said during a deposition that the charges against John Geoghan ''are false.''

In a testament to the siblings' devotion to each other, John Geoghan on Oct. 1, 2002, defied prison guards who had revoked his telephone privileges and called his sister.

Prison guards had deleted Catherine Geoghan's telephone number from the database of numbers preapproved for inmates to call. But John Geoghan eluded that restriction by having another inmate add her telephone number back to the database under a pseudonym. The fictitious name he picked: Catherine Lord.

Catherine Geoghan was a witness to one episode of abuse, in which her brother said a prison guard intentionally ''slammed'' into him in the Concord state prison visitors center, according to John Geoghan's written account of the Sept. 5, 2002, incident.

Prison guards, in disputing Geoghan's version of the incident, concluded he had lied and disciplined him by revoking his telephone privileges for six weeks -- all the while refusing requests from Geoghan and his lawyer to allow Catherine Geoghan to testify as a witness, according to Department of Correction documents on the disciplinary action.